Electrostatic field detection method for determining whether apparatus is properly grounded

ABSTRACT

A personal electrostatic safety monitor comprising an electronic circuit which continuously monitors electrostatic energy and similar forms of electrical energy for the purpose of human safety. The electronic circuit employs lights, buzzers or other suitable alerting devices to provide personnel with a warning of conditions which create a hazard or nuisance with regard to static electricity or other forms of electrical energy.

United States Patent [19] Hanna 1 1 Apr. 15, 1975 [76] Inventor: Harry A. Hanna, 3826 38th St., Des

Moines, Iowa 50310 [22] Filed: May 5, 1972 [2]] Appl. No.: 250,736

3,289,076 11/1966 Edis et a1. 324/725 X 3,309,690 3/1967 Moffitt 324/72 X 3,452,346 6/1969 Kupersmit 324/72 X 3,452,347 6/1969 Stimson 324/133 X 3.482.235 12/1969 Johnson 4 324/72 X 3.564,529 2/1971 Kaufman et a1. 324/72 X 3,667.036 5/1972 Scuchrnan .1 324/72 Primary Examiner-Gerard R. Strecker Attorney, Agent, or FirmZarley, McKee, Thomte & Voorhees [52] U.S. C1. 324/51; 324/72; 324/133; 340/258 D [51] Int. Cl. G0lr 29/12; G0lr 31/02; H05f 3/00 [57] ABSTRACT [58] Fleld of g' 'g' 5 336 A personal electrostatic safety monitor comprising an 4/ 258 D electronic circuit which continuously monitors electrostatic energy and similar forms of electrical energy [56] References cued for the purpose of human safety. The electronic cir- UNITED STATES PATENTS cuit employs lights, buzzers or other suitable alerting 2,109,189 2/1938 Bly 324/52 X devices to provide personnel with a warning of condi- 2,59 ,893 /195 st r 324/72 X tions which create a hazard or nuisance with regard to 3,009,099 11/1961 Muller 324/52 ti l t i it r other forms of electrical energy. 3,029,383 4/1962 Douglas et a1. 324/72 X 3.204.183 8/1965 Hasenzahl 324/725 X I Claim, II Drawing Figures /%sf7'zam 4 5 V 60/. fimp. l/f Qmp 1 7514090 har /m2 9 t A g. fled/n 0 Z7 fygr /3 //;//6d7br6 /z:a/)

FATENTEUAPR 1 SE75 sumaoig ELECTROSTATIC FIELD DETECTION METHOD FOR DETERMINING WHETHER APPARATL'S IS PROPERLY GROI'NDEI) The human cmironment has been detrimcntally al tered by the expanded use of electrical power and by the increased human exposure to substances or matcri als which are sensiti\e to electricity in a detrimental way. Electrical fields are commonly present in the human environment and may be caused by improperly operating electronic devices or electric equipment. improperly grounded electrical equipment. by static electricity generated by the movements of personnel. or by atmospheric electricity. It is extremely important that the presence of the electrical fields be detected to prevent human injury and property damage.

The personal electrostatic safety monitor of this invention is a self-contained. battery operated. portable safety appliance which is capable of being hermetically sealed in an operatne condition as long as the internal battery power supply is energized. The size. weight and shape of the invention is such that it can be conveniently handled. carried. or worn as a part of a uniform or clothing. or held in a shirt pocket. or attached to a belt or lapel. Whatever the source of electrical energy involved. the various methods of application of the basic circuit and its modifications will aid in providing an interpretation of these electrical fields in terms of the safety hazard they represent. Examples of such applications are described as follows:

The circuit may be so constructed and adjusted to a predetermined threshold of electrical energy detection that is arbitrarily set as a maximum safe limit in regard to the ignition sensitivity of specified categories of flammable or explosive substances such as primary of high explosives. gasoline or other petroleum products. organic solvents. anesthetic/oxygen mixtures. or dust including coal dust or saw dust. as well as many other substances.

The circuit may be so constructed and adjusted to a predetermined threshold of electrical energy detection that is arbitrarily set as a maximum safe limit in regard to the field of medical electronics. Such safe limits may correspond to the accidental electrocution of cardiac patients due to stray discharges of static electricity into cardiac probes. or the malfunctioning of cardiac elec tronic equipment or other medical electronic equipment. or improper grounding of equipment. or dangerous high-frequency radiation from electronic equip ment. etc.

The circuit may be so constructed and adjusted to a predetermined threshold of electrical energy detection that is arbitrarily set as a maximum safe limit in regard to personal electric shock.

The circuit may be so constructed and adjusted to a predetermined threshold'of electrical energy detection that is arbitrarily set as a maximum safe limit in regard to the accidental damage of delicate electronic equipmentor components. log ic circuitry or computer memories or circuitry which may be adversely affected by discharges of static electricity or related forms of elec trical energy.

The circuit may be so constructed and adjusted to a predetermined threshold of electrical energy detection that will provide a convenient and reliable indication of the grounded condition of electric or electronic equipment utilizing alternating current. high-frequency current. or equipment operated from alternating current supply lines.

As presented by the above partial listing of applications. the circuit and its modifications may be used to improve human safety by providing an adtance warn ing to concerned personnel of impending hazards or nuisances due to the presence of static electricity or other inadvertent. stray. or uncontrolled forms of elec trical energy.

The scope of this specification includes the circuit and \arious modifications to the circuit as described herein. as well as the concept of its application. or method. The invention is embodied by both the design of the circuit and the method of application.

The working of the electronic circuit employs a unique and novel concept of the "hazard value" of an electrostatic charge or other form ofstray. transient. or incidental electrical energy. Such hazard value is not directly related to any one electrical parameter such as voltage. current. field strength. or quantity of charge. Hazard value is here described as a combination of factors which together define the circumstances by which electrical energy is capable of igniting flammable or explosive materials. causing electric shock. or causing damage to sensitive materials or electronic compw nents.

These diverse factors which cause or contribute to the aforementioned hazards are most accurately gauged by measuring the rate ofchange of electrostatic field strength. Since the primary purpose of the invention is its use as a safety appliance for personnel. its primary function is therefore to monitor the rate of change of electrostatic field strength surrounding the human body. The rate of change of electrostatic field strength is used as an indication of hazardous conditions and it is related to such factors as:

l. The generation or accumulation of static electricity or related forms of electricity.

2. The extend or magnitude of the electrostatic charge or electrical generation.

3. The physical distance between the charged object.

surface or source of electrical generation and a reference point determined by the location or use of the invention.

4. Physical motion of the source of electrical energy or physical motion of the point of reference relative to the source.

5. The rate of physical motion.

6. Contact of personnel with electrical ground or electrically grounded objects.

7. The resistance of the ground path.

8. The frequency of alternating electrostatic fields which are changing in regard to amplitude or polarity.

Although the primary function of the invention is the monitoring of the electrostatic fields associated with the human body and the interpretation of the hazard thereof. an important secondary function of the invention is the monitoring of electrostatic fields associated with electric equipment. electric conductors. and other sources of electrical energy including atmospheric electricity. In this respect. the inventions sensitivity to the rate of change of electrostatic fields is used to advantage in the following manner. Modifications of the basic circuit may be used to identify un-grounded or improperly grounded electric or electronic equipment which is operated from alternating current supplied or which utilize alternating or high-frequency current. Such improperly grounded equipment will radiate an alternating electrostatic field which is easily detected by the invention. Improperly grounded equipment in many cases is a safety hazard and may ha\e the capability to cause dangerous electric shock or to cause the ignition of sensitive flammable or e\plosi\e substances. Properly grounded equipment will in general not radi ate an alternating electrostatic field.

The principle of operation for this feature of the inn \ention is based on the fact that most electric or elec tronic equipment. particularly equipment designed for use in hazardous eniironments. is enclosed in a metallie or otherwise conductive case. cabinet. or chassis which is connected to ground. generally through the third-wire electrical ground oftlie currennt supply. The grounded ease. cabinet. or chassis will prevent radiation of electrostatic fields at the power line frequency. e.g.. (it) cycles. An ungrounded or improperly grounded case. cabinet. or chassis will allow electrostatic radiation which can he conveniently detected by the invention. Such use ofthe imention has the advantage that it does not require actual physical connection for measurements nor an earth-ground reference point as required by conventional ohmeter and voltmeter measurements.

Another use of the invention is to provide utility work crews with a warning of highvoltage power lines or cables. or to determine if a power line or cable has been shut off without recourse to other conventional methods of current or voltage measurement. The invention is sufficiently sensitive to high-voltage alternating electrostatic fields that it can detect the power flowing through high-voltage utility lines at distances of several yards. The invention may also warn of accidental contact of tools. equipment. or vehicles to high-voltage power lines and aid in the prevention of accidental electrocution of personnel in work crews.

Another use of the invention is to provide a warning of the approach of electrical storms or atmospheric electrical disturbances. or high atmospheric potential gradients. Such use may be beneficial for explosive industries. biasing operations. etc.

Therefore. it is a principal object of this invention to provide a personal electrostatic safety monitor.

A further object of this invention is to provide a method by which hazardous conditions are identified in terms of the rate of change of electrostatic field strength.

A further object of this invention is to provide personnel with a warning of the existence of hazardous conditions before these conditions can cause an accident or become a nuisance in respect to the generation or accumulation of static electricity and similar forms of stray. transient. or incidental electricity.

A further object of this invention is to provide an electronic circuit to be used as a personal electrostatic safety monitor which is small in size. portable. convenient to use. reliable. and capable of being hermetically sealed.

A further object of this invention is to provide a means of identifying ungrounded or improperly grounded electric equipment or electronic equipment. which is operated from alternating current sources of power.

A further object of this invention is to provide a per sonal electrostatic safety monitor which has an ex tremcly low power drain from its internal battery supply during normal operation to allow a continous monitoring for an extended period without battery change or recharging.

A further object of this invention is to provide a warning of high-voltage alternating current power lines or cables.

A further object of this invention is to provide a warning of the accidental contact of tools. equipment. or vehicles to high-voltage alternating power lines or cables.

A further object of this invention is to provide a warning of the approach of electrical storms or the build-up of high atmospheric potential gradients.

A further object of this invention is to provide a means of tracing hidden wires or conductors carrying alternating current.

A further object of this invention is to provide a means of identifying wires or conductors by virtue ofan alternating tracer current.

These and other objects will be apparent to those who have an understanding of the discipline of electrostatics and a knowledge of the electrical hazards and nuisances encountered in various occupations or emironments wherein electrical power and electrical equipment is utilized. or where there is human exposure to electricity sensitive. flammable or explosive materials.

This invention consists of the method of application combined with the electronic circuit consisting of the construction. arrangement. and combination of its parts. whereby the objects contemplated are attained as hereinafter more fully set forth. specifically pointed out in the claims. and illustrated in the accompanying drawings in which:

FIG. 1 is a simple illustration of electrostatic principles upon which the invention is based;

FIG. 2 is a further illustration complementing FIG. I

and also showing electrostatic principles upon which the invention is based;

FIG. 3 is an electrical schematic showing a portion of one embodiment of the invention;

FIG. 4 is a graph representing current and voltage characteristics of the circuit of FIG. 3;

FIG. 5 is a graph representing the response of the circuit to external electrostatic fields;

FIG. 6 is a block diagram of one embodiment of the invention according to functions of the various parts;

FIG. 7 is an electrical schematic showing a portion of an alternate circuit also embodying the invention;

FIG. 8 is a complete detailed schematic of a working prototype of the invention;

FIG. 9 is an illustration of further electrostatic principles upon which a modification of the electronic circuit is based; and d FIG. 10 is a complete detailed schematic of a modification of the electronic circuit to be used for a specific purpose as described in the text.

FIG. II is a perspective view of a small housing enclosing this invention.

FIGS. 1 and 2 illustrate the method by which an electronic circuit is used to provide a warning of hazardous conditions by monitoring the electrostatic field surrounding the human body. These figures represent generalized cases only. Further illustrations which include specific details and values are beyond the scope of this specification. However. the validity of the application of these two general cases to the evtreme majority of conceivably hazardous situations involving electricity can be attested to by persons having a knowledge of the subject. and it can also be demonstrated mathematically.

It should be noted that the polarities shown in the illustrations. as well as the directions of current flow. may be reversed without contradicting the statements of the general cases.

Referring now to FIG. I. this illustrates the capacitive coupling of electrical energy. In terms ofcleetrostatics it is described as charging by induction. An object I is shown within the electrostatic field of some remote object 2. The field is represented by the dotted lines 3. The field causes a separation of electric charges in object l as shown. This separation of charges constitutes a movement of electronic charges within object I which is the definition of an electric current. The current is termed an induced current. This induced current exists only while the field strength is changing in value and its magnitude is directly proportional to the rate of change of the field. The invention 4 is in very close proximity to object I. The invention 4 will experience a portion of the field affecting object 1 and will also experience a proportionally induced current. The function of the invention 4 is to provide a response to induced electric currents above a specified threshold value. Therefore whenever the transfer of energy by the field 3 reaches a critical value. as determined by the adjustment of the response of the invention 4. a warning indication will be obtained. It may be assumed that object I is a person and that invention 4 is the electronic circuit or some modification of the electronic circuit given in this specification. The invention 4 may be in the shirt pocket of the person represented by object 1. carried by hand. etc.

Other considerations pertaining to the case of FIG. I which further illustrate the value of utilizing the rate of change of electrostatic field strength as an index of hazard can he explained on the basis that the ignition of electrically sensitive materials is generally regarded to have a thermal mechanism which is proportional to electrical power. as

where.

P power in watts E potential in volts current in amperes Since FIG. 1 represents the capacitive transfer of energy, the quantity of electricity involved is given by.

where.

q quantity of charge in coulombs C capacitance in farads However. power. the destructive element. is related to current in E0 l and it is therefore a parameter of q by the definition.

where.

t= time d a small change in Therefore the development of power in the capacitive transfer of energy is a function of time. more spe cifically. the rate of change of q with time. Since by Coulomb's Law an electrostatic charge. q. is attended by a proportional electrostatic field. the monitoring of the rate of change of electrostatic field strength provides an index of the destructive capability of electrical energy. or power. The same analysis also applies to the hazard ofelectric shock and damage to electrically sensitive materials. substances or objects.

As shown in the illustration of FIG. I and the accompanying text. the method as previously described is accomplished. namely. a warning is elicited of hazardous conditions as defined by the rate of change of electrostatic field strength above a specified critical value. The critical value is concerned with the ignition energy of the category of materials exposed to the situation of FIG. I. or other factors which may be evaluated for specific applications.

Referring now to FIG. 2 an object I is shown hich is not within the electrostatic field of another object but which is receiving an electric current directly at some point A as shown. The currents may be the result of contact with some material or substance which creates static charges at the point ofcontact. A. or. the current may be the result of contact with some material or substance which is capable of delivering an electric current. An electrostatic field 3 will surround object I and it will change in value at a rate which is proportional to the current flowing into object 1. The invention 4 will experience a portion of the electrostatic field 3 and it will also experience a proportionally induced current.

The case of FIG. 2 may be termed charging by contact" and it is a case in which a quantity of charge is flowing into or possibly out of an object I having an electrical capacity. C. The energy received or delivered by a charged capacitance is expressed as.

where.

W energy in joules However. as previously stated in the description of FIG. I. it is electrical power which is the prime consid eration. not energy. Power is related to energy as.

P Mr

Again time appears in the denominator and establishes a rate of change. W may change with time as either or both of its parameters change with time: tlC/(lt or (IE/(h. but in each case the attending electrostatic field will change proportionally.

Referring now to FIG. 3. the induction plate 5 is a conductor in the form of a plate. wire. rod. etc. which is electrically insulated from the electronic circuit. The function of the induction plate 5 is to make the input of the electronic circuit non-directional in terms of its sensitivity to external electrostatic fields. External electrostatic fields create a potential on the induction plate 5 which is capacitively coupled through capacitors 6 and 7 to the positive and negative charge preamplifiers. transistors 8 and 9. respectively. The capacitive charge coupled to transistors 8 and 9 results in a current flow in the base leads of these transistors which is proportional to the rate of change of external electrostatic fields. The transistor 8 is an NPN device at zero base bias and it is responsive to positi\c currents. The transistor 9 is a PNP device and is is responsive to negative currents. Balance of the response of the two transistors is aehie\ ed by the degree of capacitive coupling 6 and 7 to the induction plate 5. Transistors 8 and 9 are directly coupled in Darlington fashion to the current ainplifier. NPN transistor 10. The resistor. R is not a real part of the electronic circuit but represents the input impedance of the following amplifier states which are not shown in this illustration The significant features ofthis part of the electronic circuits are: a sensitivity to both positive and negative currents. zero base bias on all transistors in the normal or idling state causing collector cut-off and essentially zero collector current flow for prolonged battery life. and a means of estah lishing a circuit response set at a preset threshold. The preset threshold corresponds to the degree of capacitive' coupling 6 and 7 of the base leads of transistors 8 and 9 to the induction plate 5. and also to the physical size and position of the induction plate 5 with respect to the rest of the circuit. Since the transistors have zero base bias. evternal electrostatic fields must create a base potential above the base conduction threshold of the transistors to cause collector current. This potential for bipolar transistors is generally on the order of a few tenths of a volt.

This principle is illustrated in FIG. 4. a graph which shows the general form of the function of collector cur rent l,.. \s. base voltage The dotted line. V,. indicates the conduction threshold for bipolar transistors. Potentials below this value. V,. have no effect on collector current. The insensitivity of the circuit to externally applied potentials below this value prevents the circuit from responding to electrostatic fields of insignificant strength FIG. 5 is a graph which illustrates a principle utilized by the invention to conserve battery power. The dotted line represents a typical linear amplifier function in terms of total collector current. 1,. and applied external field strength. As shown. a linear amplifier arrangement would provide a minimal response to minimal fields above the v, threshold. Fields of greater strength would elicit an amplifier response of greater collector current. Since it is desired to employ a circuit which in dicates only two conditions. safe or unsafe. the graduated response represented by the dotted line is unsatisfactory. Therefore. positive regeneration or feedback is used to tailor the circuit response as represented by the solid line. This function shows only two circuit states: off or on. with no graduation between. The off on states correspond to safe and unsafe.

H0. 6 is a block diagram ofthe basic circuit arrange ment according to function. The functions denoted by numbers 5 through [0 were previously explained in FIG. 3. Continuing with the explanation from the current amplifier 10. the voltage amplifier 11 has sufficient output for operating the driver 13 and the feed-back amplifier 12. The output of the feed-back amplifier 12 is coupled back to the input of the current amplifier 10 to establish a circuit response as illustrated by the solid line in FIG. 5. While the regenerating feature of the feed-back amplifier drives the amplifiers l0 and I1 into saturation. the driver amplifier 13 is also saturated. providing power to the indicator circuit 14. This circuit may utilize lights. buzzers. or other suitable alerting devices to attract the attention ofthe person using the invention. The important features ofthe invention which (ill are illustrated by the block diagram are a tailored response which consists of two states. off and on; and a circuit insensitivity to marginal values ofexternal electrostatic field strength.

FIG. 7 is a complete. working electrical schematic of a prototype of the invention which utilizes at least all of the features previously explained. External electro static fields changing at a rate sufficient to define a hazardous condition cause an induced current from the induction plate 5 through capacitors 6 and 7 to the base leads of transistors 8 and 9. The outut of either transistor 8 or 9. depending on whether the induced current is positive or negative. respectively. is coupled to the current amplifier 10 and the voltage amplifier II. The \oltage amplifier 11 is a PNP bipolar transistor with collector loading. Transistor 11 is also normally in the cut-off condition. The voltage appearing across the 10K ohm collector load resistor 15 is coupled to the NPN bipolar transistor. driver amplifier 13 through the HJK ohm base current limiting resistor 17. The voltage across resistor 15 is also coupled to the feedback am plifier 12. an NPN transistor which provides regenerative current to the base of current amplifier l0. Coupling from resistor 15 to transistor 12 is through a 4.7 uF capacitor 18 and across a 3.9 meghom base return resistor 16. Capacitor 18 blocks DC current from the base of transistor 12 and the resistor 16 insures that transistor 12 remains normally in a cut-offcondition. In addition, resistor 16 and capacitor 18 form an RC net work giving the circuit a time constant of approximately 6 seconds. This feature acts as a one-shot multi vibrator which keeps the circuit in a saturated or on state for approximately 6 seconds regardless of the duration of the signal from the positive and negative preamplifiers. 8 and 9, respectively. As a result the circuit gives a 6 second indication to very fast transients of external fields.

As feedback is established in the circuit. the driver transistor 13 goes into saturation and provides power to the multivibrator circuit which is formed by the.

MOS FET enhancement transistors 27. 28. 29 and 30. These four transistors are in a conventional freerunning circuit with a time constant established by the 0.5 megohm resistor 31 and the 4.7 uF capacitor 32. The multivibrator frequency is approximately 3 pps. The square wave output of the multivibrator is coupled to the secondary driver. NPN transistor 25, through a 1.0 megohm base current limiting resistor 26. Power for the portion of the circuit up to. but not including transistor 25, is obtained from the 5.4 volt mercury battery 19.

The secondary driver transistor. NPN bipolar transistor 25. goes in and out of saturation with the square wave output ofthe multivibrator and sends a 3 pps collector current through the 22 ohm current limiting resistor 24 and the light-emitting diodes 21 and 22, as well as the miniature buzzer 23. Power for these indicators is taken from the 5.4 volt mercury battery 20.

Important features of this prototype circuit are: essentially zero collector or drain current for all transistors during idling. a sensitivity to very high frequencies of external electrostatic fields. a sensitivity to very fast transients of external electrostatic fields. a controlled and sustained unsafe indication. lower power consumption. a low number of passive components. and a tailored response giving precisely defined states of off and FIG. 8 is an electrical schematic representing a modification of the circuits of FIGS. 3 and 7. H6. 8 shows a pair of MOS FET enhancement transistors 34 and 35 used as the positive and negative preamplifiers. Transistor 34 is an N-channel device for positive inputs and transistor 35 is a P-channel device for negative inputs. These transistors have a high input impedance which is advantageous for preamplifier use. They are shown coupled in complimentary configuration to the current amplifier. bipolar transistor [0. R again represents the input impedance of subsequent amplifier stages not shown in FIG. 8. The diodes 36 and 48 provide a high value of gate return resistance and some degree of overload protection. The diodes 36 and 48 stabilize transistors 34 and 35 and prevent drift. The resistance of the diodes is very high due to the fact that they are reverse biased as shown.

The important features of this circuit modification are: a very high input impedance. typically 10'" to it) ohms; an extended time constant of the preamplifier circuitry; and improved temperature stability.

FIG. 9 represents a simplified diagram illustrating the electrostatic principle used to determine the grounded condition of electric or electronic equipment operated from conventional ()0 cycle current supply lines. The generator 37 is an electric power utility generating station. The wires 49 and 50 are transmission lines on utility poles. Line 50 is common with earth ground and line 49 is referred to as hot. The generator output is grounded at 38, the power station ground. The power receiving end which is represented by the resistor 39 is grounded also at 38. earth ground. The resistor 39 is assumed to be some electrical equipment utilizing power. The dotted line 40 represents a metal cover. case. or chassis around the electric equipment. If this is properly grounded. no 60 cycle electrostatic field will be radiated from the equipment 39. The grounded case. or chassis provides Faraday screening. if proper grounding is not established. a field will be radiated from the electric equipment 39 as represented by the arrow 41. The invention may be used to detect the rediated field thereby determining if the electric equipment 39 is properly grounded.

FIG. 10 is an electric schematic of a modification of the basic circuit to improve its performance for the specific purpose of detecting radiated electrostatic fields of alternating current at the power-line frequency for the purpose as described above in FIG. 9.

Since alternating electrostatic fields are being de' tected. only one preamplifier stage 43 is required which may amplify either positive or negative fields. The figure illustrates an N-channel MOS FET of the enhancement type as preamplifier 43. This transistor amplifies the positive side of the input signal. Preamplifier 43 is coupled in Darlington fashion to the NPN driv er. hipolar transistor 44. Driver 44 operates the lightemitting-diode 45 to indicate the detection of an AC electrostatic field. The entire circuit has essentially zero collector and drain current in the normal state for prolonged battery life. A feed-back circuit and amplifier is not required. The diode 47 in the gate circuit of transistor 43 is reverse biased and provides a high im pedance ground return to the gate for stabilization. A P-channel MOS FET may also be used in conjunction with a bipolar transistor for the same function. The induction plate 5 and coupling capacitor 42 couple the external AC field to the preamplifier 43. Power for the circuit is taken from the 5.4 volt mercury battery 46.

Thus it can be seen that the electronic circuits and their modifications as well as the method of employing these circuits as described will accomplish at least all of the stated objectives.

With the personal electrostatic safety monitor. it is possible for persons to obtain warnings of impending hazards or nuisances due to static electricity and other forms of electrical energy.

1 claim:

I. The method of detecting the grounded condition of electric or electronic equipment or apparatus enclosed by a conduction body by detecting the presence of a low voltage alternating electrostatic field radiating from the conductive body housing the equipment or apparatus comprising the steps of;

placing an induction plate means adjacent the conductive body within the alternating electrostatic field. if present.

capacitively coupling the induction plate means to a preamplifier stage. preamplifying the input signal and supplying the preamplified signal to a driven stage.

and connecting a signal emitting means to said driver stage to indicate the detection of the field. the degree of capacitive coupling of said preamplifier stage to the induction plate means and the size and position of said induction plate means with respect to the remainder of said circuit being such that the I circuit is activated at a low voltage value. and determining from the detection of said field and the energizing or nonenergizing of the signal emitting means if the conductive enclosure is properly grounded. energization of said signal emitting means indicating that the conductive enclosure is not properly grounded. 

1. The method of detecting the grounded condition of electric or electronic equipment or apparatus enclosed by a conduction body by detecting the presence of a low voltage alternating electrostatic field radiating from the conductive body housing the equipment or apparatus comprising the steps of: placing an induction plate means adjacent the conductive body within the alternating electrostatic field, if present, capacitively coupling the induction plate means to a preamplifier stage, preamplifying the input signal and supplying the preamplified signal to a driven stage, and connecting a signal emitting means to said driver stage to indicate the detection of the field, the degree of capacitive coupling of said preamplifier stage to the induction plate means and the size and position of said induction plate means with respect to the remainder of said circuit being such that the circuit is activated at a low voltage value, and determining from the detection of said field and the energizing or nonenergizing of the signal emitting means if the conductive enclosure is properly grounded, energization of said signal emitting means indicating that the conductive enclosure is not properly grounded. 